9.2. Primary versus Secondary Storage¶
Computer storage devices are typically classified into primary storage or main memory on the one hand, and secondary storage or peripheral storage on the other. Primary memory usually refers to Random Access Memory (RAM), while secondary storage refers to devices such as hard disk drives, solid state drives, removable “USB” drives, and DVDs. Primary memory also includes registers, cache, and video memories, but we will ignore them for this discussion because their existence does not affect the principal differences between primary and secondary memory.
As of the 2020’s, typical home computer memory sizes, speeds, and prices have become fairly stable in comparison to prior decades. Table 9.2.1 shows some representative pricing for various years from 1996 to 2025. Early years in this range saw steady decreases in prices (and consequently, increases in sizes that typical users would have). But that is not so true now.
As of 2025, both hard disk drives and solid state disk drives are widely used for secondary storage, with roughly a factor of five (or, half an order of magnitude) difference in unit price. In contrast, RAM is about two orders of magnitude more expensive per unit of storage compared to SSD storage, and 2.5 orders of magnitude more expensive compared to HDD.
HDD and SSD has another advantage over RAM memory, in that it is persistent, meaning that data are not erased from the media when the power is turned off. In contrast, RAM used for main memory is usually volatile—all information is lost with the power.
In exchange for reduced storage costs and persistence, secondary storage devices pay a penalty in terms of increased access time. While not all accesses to disk take the same amount of time (more on this later), the typical time required to access a byte of storage from a HDD in 2025 is around 9 ms (i.e., 9 thousandths of a second). This metric is sometimes referred to as latency. This is roughly the same as it has been for the past 20 years. This might not seem slow, but compared to the time required to access a byte from main memory, this is fantastically slow. Typical access time from standard personal computer RAM in 2025 is about 10-15 nanoseconds (i.e., 10-15 billionths of a second). (Again, this has been roughly the same for many years.) Thus, the time to access a byte of data from a HDD is about six orders of magnitude greater than that required to access a byte from main memory. While disk drive and RAM access times have decreased over the past 30-40 years, they have done so at roughly the same rate. The relative speeds have remained the same for over several decades, in that the difference in access time between RAM and a disk drive has remained in the range between a factor of 100,000 and 1,000,000.
SSD latency times are considerably less than HDD latency times. For the SSD, this is something around 100 microseconds, or 100 times faster than for a typical HDD on average. But that is still about four orders of magnitude slower than RAM latency time.
To gain some intuition for the significance of this speed difference, consider the time that it might take for you to do an internet search for disk drives, then click on a link and load the page. Call this your “primary memory” access time. If it takes you about 10 seconds to perform this access, then an access taking a million times longer would require months. In this analogy, SSD latency time is still a matter of weeks.
It is interesting to note that while processing speeds have increased dramatically, and hardware prices have dropped dramatically, disk and memory access times have improved by less than an order of magnitude over the past 30 years. However, the situation is really much better than that modest speedup would suggest. During the same time period, the size of both disk and main memory has increased by over three orders of magnitude. Thus, the access times have actually decreased in the face of a massive increase in the density of these storage devices.
Due to the relatively slow access time for data on disk as compared to main memory, great care is required to create efficient applications that process disk-based information. The million-to-one ratio of HDD access time versus main memory access time makes the following rule of paramount importance when designing disk-based applications:
Minimize the number of disk accesses!
There are generally two approaches to minimizing disk accesses. The first is to arrange information so that if you do access data from secondary memory, you will get what you need in as few accesses as possible, and preferably on the first access. File structure is the term used for a data structure that organizes data stored in secondary memory. File structures should be organized so as to minimize the required number of disk accesses. The other way to minimize disk accesses is to save information previously retrieved (or retrieve additional data with each access at little additional cost) that can be used to minimize the need for future accesses. This requires the ability to guess accurately what information will be needed later and store it in primary memory now. This is referred to as caching.
